Current prosthetic ankle-foot systems are generally aligned for operation as fixed mechanical structures comprising elastic and deformable elements designed to provide stability during standing and walking and to return energy for propulsion into the swing phase of the walking cycle. However, such a device is often uncomfortable for the user whilst standing and walking on ramps and stairs and walking at different speeds. Users have also experienced knee instability and difficulty in maintaining forward motion during roll-over of the foot while standing and walking on ramps and stairs, with consequent impairment of efficiency. These difficulties are particularly important for transfemoral amputees whose stance phase action is normally compromised by significantly reduced knee flexion and extension which would otherwise assist shock absorption and forwards propulsion during the stance phase.
Another aspect of ankle-foot function and transfemoral amputee locomotion relates to the way in which a typical known prosthesis hinders the amputee, resulting in poor body posture for certain locomotion activities such as ascending and descending stairs and ramps, which diminishes the potential for application of voluntary control, particularly user-generated hip extension torque. The poor posture described is largely caused by inappropriate stiffness and range of motion at the ankle which does not allow the body centre of mass to pass easily over the ankle. Consequently, amputees sometimes have to adopt unnatural compensating actions.
An ankle joint mechanism allowing dynamic hydraulic control of the angular position of a prosthetic foot with respect to a shin component is disclosed in Mauch Laboratories, Inc., Hydraulik Ankle Unit Manual, March 1988. The shin component is attached to a vane piston housed in a fluid-filled chamber with a concave part-circular lower wall. A gravity-controlled ball rolls forwards and backwards on the wall according to the orientation of the foot to open or close a bypass passage in the piston. As a result, dorsi-flexion of the mechanism is prevented when the shin component is vertical, largely irrespective of whether the foot is horizontal or inclined downwardly or upwardly. Such a prosthesis also suffers partly from the disadvantages described above.
In US2002/0138153 (Koniuk) a self-adjusting ankle with a similar function is disclosed. This unit switches between two damping resistance levels, the switch between the two damping levels being triggered by detection of a shin pylon reaching a vertical orientation. The second damping level is set effectively to prevent pivoting of the foot.
Amongst other known prosthetic ankle systems is that of U.S. Pat. No. 3,871,032 (Karas). This system contains a damping device having a dual piston and cylinder assembly with tappet return springs acting continuously to return the ankle to a neutral position. EP-A-0948947 (O'Byrne) discloses a prosthetic ankle having a ball-and-socket joint with a chamber filled with a silicone-based hydraulic substance, the joint having a visco-elastic response. In one embodiment, the chamber contains solid silicone rubber particles suspended in a silicone fluid matrix. US2004/0236435 (Chen) discloses a hydraulic ankle arrangement with adjustable hydraulic damping and resilient biasing members mounted anteriorly and posteriorly of an ankle joint rotation axis. In WO00/76429 (Gramtec), a leg prosthesis is described having an ankle joint allowing heel height adjustment by way of a hydraulic piston and linkage arrangement. Elastic components absorb shock during walking. US2006/0235544 (Iversen et al) discloses a hydraulic ankle mechanism with a rotary vane.
The electronically controlled ankle disclosed in WO2003/086245 (Martin) has a magnetorheological (MR) fluid-controlled ankle component. WO2007/027808 (Ossur) discloses an electronically controlled ankle joint in which the angle of foot springs about an ankle joint is altered by means of a motorised coupling.
It is an object of the present invention to provide a more natural function in a variety of situations.